The present invention generally relates to feedthrough capacitors. More particularly, the present invention relates to feedthrough capacitors which have been constructed in such a manner so as to substantially reduce, or even completely eliminate, the self-resonance insertion loss dip.
Feedthrough capacitors are well known in the prior art. Feedthrough capacitors are different from chip monolithic ceramic capacitors (MLCC). Rectangular chip capacitors are two-terminal devices and therefore have significant internal inductance. Feedthrough capacitors are three-terminal devices that act as transmission lines and therefore have very low internal inductance. Feedthrough capacitors are very useful for electromagnetic interference (EMI) filters in that they provide substantial attenuation over a very broad frequency range. Feedthrough capacitors can be ceramic single or multilayer, tubular ceramic, wound film types, wound impregnated paper types, thick film deposited, thin film deposited, glass ceramics and even stabilized multi-component systems. As used herein, the term “feedthrough capacitor” shall be inclusive of all of these and equivalent technologies. For simplicity, the drawings that illustrate the inventive concepts herein are shown for multilayer ceramic feedthrough capacitors. It will be obvious to those skilled in the art that these novel design principles may be adapted to all other feedthrough capacitor technologies.
A limitation to all prior art feedthrough capacitors is their tendency to exhibit one or more self-resonant dips. These self-resonant dip characteristics are not completely understood, but at least in part has to do with the interaction of the internal electrode plates and their own self-inductance in a transmission line configuration. Self-resonant dips are a very undesirable characteristic of feedthrough capacitors because they reduce the amount of effective attenuation at the resonant dip frequencies. Accordingly, less protection to circuits is provided from electromagnetic interference.
The inventors have noted that the higher the Quality Factor, or Q, of the capacitor (that is, the lower its internal losses), the sharper the resonant dips tend to be. A paper entitled Improved EMI Insertion Loss Test Methods, Equipment and Fixtures, given at the 1996 CARTS Symposium, (incorporated by reference), describes a method of fixturing various types of EMI filters and feedthrough capacitors, and includes a substantial discussion of resonant dips. An example is given of a United States defense standard, MIL-PRF-28861 (MIL-SPEC), filter that had a specification limit of 20 dB, but because of the self-resonant dip of the feedthrough capacitor, only 13 dB was achieved at this particular frequency. No manufacturer found a way to eliminate the self-resonant dip, thus the MIL-SPEC was modified to allow for the self-resonant dip. Of course, this was not ideal.
Accordingly, there is a need for a feedthrough capacitor in which the amplitude of the self-resonance insertion loss dip is significantly reduced or even eliminated. The present invention fulfills this need and provides other related advantages.